1. Technical Field
The present invention relates to a composition for forming a solid electrolyte layer, a method for forming a solid electrolyte layer, a solid electrolyte layer, and a lithium ion secondary battery.
2. Related Art
Lithium ion secondary batteries are lightweight and have high capacity. In addition, they also provide high voltage when a suitable active material or the like is used. Therefore, they have been widely applied as batteries for portable electronic devices, cameras, watches, electrical power tools, and hybrid cars, for example.
However, because of the high activity of lithium and the use of an organic electrolyte solution, the danger of ignition upon short circuit, for example, is concerned. Therefore, in the design of a lithium battery, the assurance of safety is a big issue.
As an attempt to use no electrolyte solution, a lithium ion battery using an inorganic solid electrolyte has been attracting attention (see, e.g., JP-A-2008-59843 (Patent Document 1)).
As inorganic solid electrolytes, amorphous oxides, amorphous sulfides, and the like have been found, but attention has been focused on crystalline oxides having high chemical and thermal stability, particularly lithium lanthanum titanates (LLT) having high lithium ion conductivity.
In order to obtain a solid electrolyte layer made of a lithium lanthanum titanate, generally, a bulk lithium lanthanum titanate obtained by the high-temperature, solid-phase synthesis of lithium carbonate, lanthanum oxide, titanium oxide, or the like is pulverized into a powder having a desired particle diameter, and the powder is pressed and sintered at a high temperature of a thousand and several hundreds of degrees Celsius for a long period of time (see, e.g., Patent Document 1). Thus, in the related art, the formation of a solid electrolyte layer made of a lithium lanthanum titanate includes a step of maintaining a high temperature for a long period of time.